1. Field of the Invention
This invention relates to a method for separating isobutyric acid from an aqueous hydrogen fluoride solution thereof by a liquid-liquid extraction procedure.
2. Description of the Prior Art
It is known that olefins react rapidly with carbon monoxide at low temperatures and under moderate pressure in the presence of a strong acidic media, typically representative of which is anhydrous hydrogen fluoride, concentrated sulfuric acid and anhydrous chlorosulfonic acid. The postulated carbonylation mechanism involved is conventionally referred to as the Koch reaction and is exemplified in U.S. Pat. No. 2,831,879. The reaction reportedly provides a good yield of acids only when butene or a higher olefin is used. In the case of propylene, the formation of by-products is indicated to be rampant and an exceedingly lower reaction temperature is taught as being required in order to obtain yields on the order of that observed in utilizing the higher olefins.
Noting the relative ineffectiveness of the Koch reaction for carbonylating propylene, subsequent investigators, Takezaki et al, undertook to study the use of aqueous hydrogen fluoride (HF) in order to improve yields of isobutyric acid. Their findings are given in Bull. Jap. Pet. Inst., 8, 31-38 (1966) wherein it is reported that optimum operating conditions, including the use of 80% HF-20% H.sub.2 O as the reaction medium, provided a commercially acceptable yield of isobutyric acid.
While yield of product is very important for any commercial undertaking, an equally important consideration in this instance is that the substantial amount of HF required for effecting the reaction must be recovered essentially in toto for recycling. Moreover, it must be readily recoverable in about an 80% aqueous form as employed in the reaction.
Komatsu et al, Bull. Jap. Pet. Inst., 16, 124-131 (1974) addresses this recovery aspect as it specifically applies to the Koch reaction employing a lower olefin. A procedure is accordingly outlined therein for the recovery of HF following the preparation of pivalic acid in accordance with the Koch reaction. Essentially the procedure involves hydrolyzing the carbonylation reaction mixture with 53% aqueous HF and distilling to recover anhydrous HF. Thereupon the residue in the form of an equimolar HF complex of pivalic acid is hydrolyzed using a minimum amount of water to provide two layers; the top layer being the carboxylic acid product and the other being recyclable 50% aqueous HF.
However, the foregoing procedure is not applicable to the preparation of isobutyric acid in 80% aqueous HF for a combination of factors. For one, said carboxylic acid forms directly in accordance with this reaction and exhibits complete solubility in the aqueous HF medium. Secondly, 35% aqueous HF represents a constant boiling mixture in which isobutyric acid still remains soluble. Accordingly, one is constrained to distill the reaction mixture to recover as much HF as possible then dilute with a substantial amount of water to effect phase separation. The diluted HF solution can of course be concentrated by distillation to provide the 35% aqueous HF for recycling. But such a recovery method would be too tedious and energy intensive to be practical.